1. Field of the Invention
This invention relates to vulcanizable rubber compositions which combine vulcanizable rubber with effective amounts of a complex of zinc hexasulfide and an amine and of co-agent which comprises one or more vulcanization accelerators.
2. Related Art
Complexes of zinc hexasulfide with an amine are described in a publication, "Direct Approaches to Zinc Polychalcogenide Chemistry: ZnS.sub.6 (N-MeIm).sub.2 and ZnSe.sub.4 (N-MeIm).sub.2 " by Dev, Ramli, Rauchfuss and Stern, JACS 112, 6385 (1990). The article describes the preparation and properties of the complexes, and suggests that they are "relevant to the action of zinc catalysts for the addition of polysulfur radicals to polyolefins in the vulcanization process."
Experiments in which these complexes were used as accelerators in standard rubber formulations were unsuccessful; instead, the effect of the complexes was to produce extremely slow cure (vulcanization) times, and the complexes produced the same effect as if elemental sulfur had been added. Thus, the expected "zinc catalyst" effect was not achieved and the vulcanizates required impractically long cure times.
Further experiments showed, surprisingly, that the inclusion of a co-agent, in effective amounts, gave compositions which vulcanized much faster, more quickly even than conventional formulations. Specifically, the additional presence of a vulcanization accelerator in an amount of from 0.1 to 10 parts by weight per 100 parts by weight of vulcanizable rubber, proved to be effective in producing vulcanizates with fast cure rates and superior physical properties.